It has been traditional to burn coal in the presence of excess air to generate heat that can be used directly or indirectly for the purpose of providing steam energy demands. This steam is subsequently used in industrial boilers to provide heat or in utility boilers to generate electricity. It is well known that the amount of excess air is carefully controlled, not only for combustion-zone purposes, but also to optimize the residence time and heat transfer in the convective zone of the furnace. Therefore, under these conditions, oxygen partial pressure is primarily maintained by controlling the excess air flow rate which at times can be beneficial in the combustion zone, but detrimental elsewhere in the furnace.
One type of furnace used for combusting pulverized coal comprises a furnace firebox having a primary air source and a secondary air source which provide at least a sufficient amount of air for the complete combustion of the coal. The pulverized coal is fed into the firebox suspended in the primary airstream. Typically, a nozzle having concentric cylindrical passageways is the means for injecting the primary and secondary airstreams. The primary airstream containing suspended pulverized coal is fed through the central passageway of the nozzle while the secondary airstream is injected through the surrounding, or annular, passageway.
Fireboxes are being fired with coals containing increasing ash contents due to their low price and the unavailability of low-ash coals. These high-ash coals, along with other low-ash coals which have a high tendency to slag, present a problem since large slag "chunks" can damage internals and slag buildup within the firebox requires frequent shutdowns. Slagging is the deposition on the internals of the furnace firebox of the mineral matter in the coal which has gone through a molten state during the combustion of the coal and adhered to a surface.
Attempts to solve this problem of slag deposition include cleaning coal to reduce the ash content. However, this method is very expensive since the coal must be ground extra fine to remove the ash. Also, various chemical additives can be used, either sprayed on the coal prior to combustion or injected into the combustion zone, to help reduce slagging.
U.S. Pat. No. 2,865,344 discloses adding oxygen to recycled combustion gases as a replacement for some of the air injected into the furnace in order to decrease stack heat losses and to better control furnace temperatures.
U.S. Pat. No. 3,250,236 discloses the combustion of pulverized fuel and is directed toward the production of large quantities of highly heated and high velocity products of combustion which are suitable for operating magnetohydrodynamic generators. A primary gas which typically is air and pulverized coal are mixed with preheated, oxygen-enriched secondary air to produce a homogeneous mixture before combustion. The mixing is accomplished in a particular mixing means in an injection nozzle.
U.S. Pat. No. 3,628,322 discloses delivering oxygen to a gas producer to gasify coal in order to produce fuel which is then delivered with more oxygen to a boiler furnace to generate steam.
U.S. Pat. No. 3,699,903 discloses a method in which oxygen is used alone, preferably in stoichiometric quantities, as the secondary or combustion gas, and with only enough primary air for fuel feed, adequate gas velocities, fuel turbulence and distribution in the furnace firebox.
U.S. Pat. No. 4,052,138 discloses a method of firing coal-powered boilers to produce heat to operate the boiler and also to produce a usable fuel gas. The method consists of burning pulverized coal in the presence of varying proportions of steam and oxygen. The boiler is over-fired with coal, but with an undersupply of oxygen, in order that the total available heat content of the coal is divided between heat released by the incomplete combustion of the coal and heat content contained in combustible flue gases.
U.S. Pat. No. 4,261,167 discloses gasifying a carbonaceous fuel at relatively high pressure and temperature by partial oxidation with oxygen to produce a fuel gas, the fuel gas from the partial oxidation reaction is expanded to a lower pressure in a turbine and, after removal of undesirable compounds, is supplied to a steam boiler for the production of power.
U.S. Pat. No. 4,329,932 discloses a method of burning fuel with lowered emission of nitrogen oxides which comprises feeding pulverized fuel to the main burner in a combustion furnace and additionally feeding pulverized fuel to the region of the furnace where the first fuel is about to conclude its combustion, using inert gas with or without a low oxygen content as a conveying fluid, while supplying oxygen or air to a region downstream of the region for fuel addition.
B. Ghosh, et al., Ind. Eng. Che., 47, 117-121 (1955) discuss the effect of oxygen enrichment on the ignition time of pulverized coal.
Other literature articles relating to oxygen enrichment of pulverized coal include:
A. Ivernel, Proc. Combustion Inst. European Symp. 1973, 463-468; PA0 B. Ghosh, et al., 6th Symposium on Combustion, 595-601 (1957); PA0 R. P. Weight, J. Inst. Energy, 54, 176 (1981); and PA0 S. Bandyopadhyay, et al., Combustion and Flame, 18, 411-415 (1972).